1. Field of the Invention
The present invention relates to a push-on switch in which a dome-like movable contact is inverted as a result of a depressing operation to make electrical conduction, and more particularly to a two-step push-on switch into which two or upper and lower movable contacts are incorporated.
2. Description of the Prior Art
In a two-step push-on switch into which two or upper and lower movable contacts are incorporated, usually, circular dome-like plate springs are used as the movable contacts. In this case, in order to increase the operation stroke, the movable contacts must have a large external shape, thereby producing a problem in that the whole size of the switch is increased. In a two-step push-on switch, the upper movable contact itself conducts an inverting operation, and a central portion is displaced to a position where the lower movable contact conducts an inverting operation. Therefore, a stress produced in such an upper movable contact during an inverting operation is larger than that produced in a movable contact of a usual one-step push-on switch or a lower movable contact of a two-step push-on switch. Consequently, there arise further problems in that failures such as cracks or deflection (creep) easily occur, and that the life number of ON/OFF operations is reduced. During inverting operations in a two-step push-on switch, deformed shapes of upper and lower movable contacts are similar to each other, and hence it is difficult to definitely obtain senses of two-step operations. Therefore, it is required to employ a structure in which a support member is disposed as another member in a central area between the upper and lower movable contacts, and adjustment must be conducted so as to obtain excellent senses of operations. This structure produces a problem in that the number of parts is increased. As a countermeasure, it has been proposed to realize a switch structure in which a dome-like plate spring having a shape other than a circle is used as an upper movable contact, and a stress during an inverting operation of the upper movable contact is relaxed while ensuring the operation stroke of the movable contact, and the installation area can be reduced.
A specific example of such a two-step push-on switch in which a dome-like plate spring having a shape other than a circle is used as an upper movable contact is disclosed in Japanese Patent Application Laying-Open No. 2003-7168. The proposed switch comprises: a housing having an accommodating portion; a central stationary contact which is disposed on an inner bottom face of the accommodating portion of the housing; a first peripheral stationary contact which is disposed outside the central stationary contact; a second peripheral stationary contact which is disposed outside the first peripheral stationary contact; a dome-like lower movable contact in which a central area of an inflated portion is opposed to the central stationary contact, and a peripheral edge portion is always in contact with the first peripheral stationary contact; and a dome-like upper movable contact which has a diameter larger than the external shape of the lower movable contact, and in which a central area of an inflated portion is opposed to the lower movable contact so as to cover the lower movable contact, and a peripheral edge portion is always in contact with the second peripheral stationary contact. A plurality of arms are crosswise elongated from the upper movable contact so as to be continuous to the invertible inflated portion. In the four diagonal corners of the housing, accommodating recesses in which the arms of the upper movable contact are respectively accommodated in a bendable and stretchable manner are formed so as to be continuous to the accommodating portion. The second peripheral stationary contact which is always in contact with at least one of the arms is placed in the accommodating recesses.